In loudspeaker device provided with a box and a loudspeaker unit which is attached to an inner wall surface of the box, a reduction of box volume leads to a worsening of an audio performance of low-frequency range sounds, since an acoustic stiffness of the loudspeaker device increases.
Conventionally, as a method to solve the above problem, there are various suggestions of loudspeaker devices in which granular or fibrous activated carbon generated from palm shell, wood, coal pitch, petroleum pitch, phenol resin, or the like is placed inside the box so that the acoustic stiffness is reduced by a physical gas adsorption action of the activated carbon and an audio range of the low-frequency range sounds is widened.
For example, a closed box loudspeaker device in which activated carbon fiber in form of cloth (a micropore diameter: Φ1.0 nm to 2.5 nm, a specific surface area: 500 m2 /g or more) generated from the phenol resin is placed inside a box is suggested. In the activated carbon fiber, nano-order pores, which is referred to as micropores, are exposed on a surface of the fiber, and moreover, there is no macropore to form a high cut filter, so that a physical gas adsorption effect is also effective in high frequency range sounds of 100 to 200 Hz or more. Accordingly, the closed box loudspeaker device which has the effect of widening the audio range of the low-frequency range sounds can also be achieved with a small-sized box which has the audio range of the low-frequency range sounds, whose frequency band is relatively high.
However, when the above activated carbon fiber adsorbs moisture, its gas adsorption performance is deteriorated, so that the activated carbon fiber could not be applied to a loudspeaker device such as a bass reflex loudspeaker device in which outside air flows into the box.
Thus, there is a suggestion of a loudspeaker device in which activated carbon fiber wrapped in a shielding member, which is made of a plastic film such as polyvinyl chloride, vinylidene chloride, or the like, is placed in a box to prevent the deterioration of the activated carbon fiber by the moisture (refer to patent document 1, for example).
Moreover, a loudspeaker system in which activated carbon (granular activated carbon made from coal), whose pore diameter is 3.6 to 10 nm and a total volume of the pores is 0.4 ml/g or more, is placed in a box is suggested as a loudspeaker system which improves a moisture resistance. Even when the activated carbon is left in an atmosphere having a temperature of 30 degrees Celsius and a relative humidity of 70% for 24 to 48 hours, its gas adsorption performance is not reduced, so that the activated carbon can be applied to a bass reflex loudspeaker system in which the outside air flows into a box (refer to patent document 2, for example).